Showing papers on "Shock tube published in 2013"

On the interaction of shock waves and sound waves in transonic buffet flow

Abstract: To support Lee's buffet mechanism model [B. H. K. Lee, “Self-sustained shock oscillations on airfoils at transonic speeds,” Prog. Aerosp. Sci. 37, 147–196 (2001)10.1016/S0376-0421(01)00003-3], the sound wave propagation in the flow field outside the separation of a transonic buffet flow at a Mach number M∞ = 0.73 and an angle of attack α = 3.5° over a DRA 2303 supercritical airfoil is determined using high-speed particle-image velocimetry. Furthermore, the shock wave is influenced by an artificial sound source which evidently changes the shock oscillation properties. The dominant buffet mechanism is shown to be a feedback loop between the shock position and the noise generation at the trailing edge of the airfoil. The sound wave propagation speed is detected by correlating the surface pressure signals and the velocity fluctuations in the flow field. The quantitative results for the natural and the artificial sound source convincingly coincide and are in good agreement with a reformulated version of Lee's ...

Ignition delay times of methyl oleate and methyl linoleate behind reflected shock waves

Abstract: Ignition delay times for methyl oleate (C 19 H 36 O 2 , CAS: 112-62-9) and methyl linoleate (C 19 H 34 O 2 , CAS: 112-63-0) were measured for the first time behind reflected shock waves, using an aerosol shock tube The aerosol shock tube enabled study of these very-low-vapor-pressure fuels by introducing a spatially-uniform fuel aerosol/4% oxygen/argon mixture into the shock tube and employing the incident shock wave to produce complete fuel evaporation, diffusion, and mixing Reflected shock conditions covered temperatures from 1100 to 1400 K, pressures of 35 and 70 atm, and equivalence ratios from 06 to 24 Ignition delay times for both fuels were found to be similar over a wide range of conditions The most notable trend in the observed ignition delay times was that the pressure and equivalence ratio scaling were a strong function of temperature, and exhibited cross-over temperatures at which there was no sensitivity to either parameter Data were also compared to the biodiesel kinetic mechanism of Westbrook et al (2011) [10] , which underpredicts ignition delay times by about 50% Differences between experimental and computed ignition delay times were strongly related to existing errors and uncertainties in the thermochemistry of the large methyl ester species, and when these were corrected, the kinetic simulations agreed significantly better with the experimental measurements

Experimental and modeling study on the pyrolysis and oxidation of iso-octane

Abstract: High pressure iso-octane shock tube experiments were conducted to assist in the development of a Jet A surrogate kinetic model. Jet A is a kerosene based jet fuel composed of hundreds of hydrocarbons consisting of paraffins, olefins, aromatics and naphthenes. In the formulation of the surrogate mixture, iso-octane represents the branched paraffin class of hydrocarbons present in aviation fuels like Jet A. The experimental work on iso-octane was performed in a heated high pressure single pulse shock tube. The mole fractions of the stable species were determined using gas chromatography and mass spectroscopy. Experimental data on iso-octane oxidation and pyrolysis were obtained for temperatures from 835 to 1757 K, pressures from 21 to 65 atm, reactions times from 1.11 to 3.66 ms, and equivalence ratios from 0.52 to 1.68, and ∞. Iso-octane oxidation showed that the fuel decays through thermally driven oxygen free decomposition at conditions studied. This observation prompted an experimental and modeling study of iso-octane pyrolysis using an iso-octane sub-model taken from a recently published n-decane/iso-octane/toluene surrogate model. The revised iso-octane sub-model showed improvements in predicting intermediate species profiles from pyrolytic experiments and oxidation experiments. The modifications to the iso-octane sub-model also contributed to better agreement in predicting the formation of carbon monoxide and carbon dioxide when compared to the recently published 1st Generation Surrogate model and a recently published iso-octane oxidation model. Model improvements were also seen in predicting species profiles from flow reactor oxidation experiments and ignition delay times at temperatures above 1000 K at both 10 and 50 atm.

A switch to reduce resistivity in smoothed particle magnetohydrodynamics

Abstract: Artificial resistivity is included in Smoothed Particle Magnetohydrodynamics simulations to capture shocks and discontinuities in the magnetic field. Here we present a new method for adapting the strength of the applied resistivity so that shocks are captured but the dissipation of the magnetic field away from shocks is minimised. Our scheme utilises the gradient of the magnetic field as a shock indicator, setting {\alpha}_B = h|gradB|/|B|, such that resistivity is switched on only where strong discontinuities are present. The advantage to this approach is that the resistivity parameter does not depend on the absolute field strength. The new switch is benchmarked on a series of shock tube tests demonstrating its ability to capture shocks correctly. It is compared against a previous switch proposed by Price & Monaghan (2005), showing that it leads to lower dissipation of the field, and in particular, that it succeeds at capturing shocks in the regime where the Alfv\'en speed is much less than the sound speed (i.e., when the magnetic field is very weak). It is also simpler. We also demonstrate that our recent constrained divergence cleaning algorithm has no difficulty with shock tube tests, in contrast to other implementations.

Shock-Tube Experiments and Kinetic Modeling of 2-Methylfuran Ignition at Elevated Pressure

Abstract: Ignition delays of 2-methylfuran were measured behind reflected shock waves over a wide range of experimental conditions: equivalence ratios from 0.25 to 2.0, average pressures from 1.25 to 10.65 bar, temperatures from 1120 to 1700 K, and oxygen concentrations up to 20%. Results show that the ignition delay decreases with increasing the pressure and decreasing the dilution ratio. For a given dilution ratio, there exists a crossover in the ignition delay time dependence upon the equivalence ratio and the crossing point shifts to the higher temperature at a higher pressure. The measured ignition delays of 2-methylfuran show good agreement with the previous data at atmospheric pressure. The 2-methylfuran model NUI_MF2 well predicts the ignition delays of 2-methylfuran at 1.25 bar but gives the underprediction when pressures are elevated to 4.25 and 10.65 bar. Sensitivity analysis identifies the importance of the reactions involving the n-butadienyl radical (C4H5-n) in the ignition process of 2-methylfuran. B...

Experiments on the Richtmyer-Meshkov Instability with an Imposed, Random Initial Perturbation

Abstract: A vertical shock tube is used to perform experiments on the Richtmyer–Meshkov instability with a three-dimensional random initial perturbation. A membraneless flat interface is formed by opposed gas flows in which the light and heavy gases enter the shock tube from the top and from the bottom of the shock tube driven section. An air/SF $$_{6}$$ gas combination is used and a Mach number $$ M = 1.2$$ incident shock wave impulsively accelerates the interface. Initial perturbations on the interface are created by vertically oscillating the gas column within the shock tube to produce Faraday waves on the interface resulting in a short wavelength, three-dimensional perturbation. Planar Mie scattering is used to visualize the flow in which light from a laser sheet is scattered by smoke seeded in the air, and image sequences are captured using three high-speed video cameras. Measurements of the integral penetration depth prior to reshock show two growth behaviors, both having power law growth with growth exponents in the range found in previous experiments and simulations. Following reshock, all experiments show very consistent linear growth with a growth rate in good agreement with those found in previous studies.

The elastic–plastic behaviour of foam under shock loading

Abstract: An experimental investigation of the elastic–plastic nature of shock wave propagation in foams was undertaken. The study involved experimental blast wave and shock tube loading of three foams, two polyurethane open-cell foams and a low-density polyethylene closed-cell foam. Evidence of precursor waves was observed in all three foam samples under various compressive wave loadings. Experiments with an impermeable membrane are used to determine if the precursor wave in an open-cell foam is a result of gas filtration or an elastic response of the foam. The differences between quasi-static and shock compression of foams is discussed in terms of their compressive strain histories and the implications for the energy absorption capacity of foam in both loading scenarios. Through a comparison of shock tube and blast wave loading techniques, suggestions are made concerning the accurate measurements of the principal shock Hugoniot in foams.

Biomechanical responses of a pig head under blast loading: a computational simulation

Abstract: A series of computational studies were performed to investigate the biomechanical responses of the pig head under a specific shock tube environment. A finite element model of the head of a 50-kg Yorkshire pig was developed with sufficient details, based on the Lagrangian formulation, and a shock tube model was developed using the multimaterial arbitrary Lagrangian-Eulerian (MMALE) approach. These two models were integrated and a fluid/solid coupling algorithm was used to simulate the interaction of the shock wave with the pig's head. The finite element model-predicted incident and intracranial pressure traces were in reasonable agreement with those obtained experimentally. Using the verified numerical model of the shock tube and pig head, further investigations were carried out to study the spatial and temporal distributions of pressure, shear stress, and principal strain within the head. Pressure enhancement was found in the skull, which is believed to be caused by shock wave reflection at the interface of the materials with distinct wave impedances. Brain tissue has a shock attenuation effect and larger pressures were observed in the frontal and occipital regions, suggesting a greater possibility of coup and contrecoup contusion. Shear stresses in the brain and deflection in the skull remained at a low level. Higher principal strains were observed in the brain near the foramen magnum, suggesting that there is a greater chance of cellular or vascular injuries in the brainstem region.

An experimental and modeling study of the shock tube ignition of a mixture of n-heptane and n-propylbenzene as a surrogate for a large alkyl benzene

Abstract: Alkyl aromatics are an important chemical class in gasoline, jet and diesel fuels. In the present work, an n -propylbenzene and n -heptane mixture is studied as a possible surrogate for large alkyl benzenes contained in diesel fuels. To evaluate it as a surrogate, ignition delay times have been measured in a heated high pressure shock tube (HPST) for a mixture of 57% n -propylbenzene/43% n -heptane in air (≈21% O 2 , ≈79% N 2 ) at equivalence ratios of 0.29, 0.49, 0.98 and 1.95 and compressed pressures of 1, 10 and 30 atm over a temperature range of 1000–1600 K. The effects of reflected-shock pressure and equivalence ratio on ignition delay time were determined and common trends highlighted. A combined n -propylbenzene and n -heptane reaction mechanism was assembled and simulations of the shock tube experiments were carried out. The simulation results showed very good agreement with the experimental data for ignition delay times. Sensitivity and reaction pathway analyses have been performed to reveal the important reactions responsible for fuel oxidation under the shock tube conditions studied. It was found that at 1000 K, the main consumption pathways for n -propylbenzene are abstraction reactions on the alkyl chain, with particular selectivity to the allylic site. In comparison at 1500 K, the unimolecular decomposition of the fuel is the main consumption pathway.

Single pulse shock tube study of allyl radical recombination.

Abstract: The recombination and disproportionation of allyl radicals has been studied in a single pulse shock tube with gas chromatographic measurements at 1–10 bar, 650–1300 K, and 1.4–2 ms reaction times. 1,5-Hexadiene and allyl iodide were used as precursors. Simulation of the results using derived rate expressions from a complementary diaphragmless shock tube/laser schlieren densitometry study provided excellent agreement with precursor consumption and formation of all major stable intermediates. No significant pressure dependence was observed at the present conditions. It was found that under the conditions of these experiments, reactions of allyl radicals in the cooling wave had to be accounted for to accurately simulate the experimental results, and this unusual situation is discussed. In the allyl iodide experiments, higher amounts of allene, propene, and benzene were found at lower temperatures than expected. Possible mechanisms are discussed and suggest that iodine containing species are responsible for t...

A miniature high repetition rate shock tube

Abstract: A miniature high repetition rate shock tube with excellent reproducibility has been constructed to facilitate high temperature, high pressure, gas phase experiments at facilities such as synchrotron light sources where space is limited and many experiments need to be averaged to obtain adequate signal levels. The shock tube is designed to generate reaction conditions of T > 600 K, P < 100 bars at a cycle rate of up to 4 Hz. The design of the apparatus is discussed in detail, and data are presented to demonstrate that well-formed shock waves with predictable characteristics are created, repeatably. Two synchrotron-based experiments using this apparatus are also briefly described here, demonstrating the potential of the shock tube for research at synchrotron light sources.

Effects of NO2 addition on hydrogen ignition behind reflected shock waves

Abstract: Ignition delay time measurements of H2/O2/NO2 mixtures diluted in Ar have been measured in a shock tube behind reflected shock waves. Three different NO2 concentrations have been studied (100, 400 and 1600 ppm) at three pressure conditions (around 1.5, 13, and 30 atm) and for various H2–O2 equivalence ratios for the 100 ppm NO2 case. Results were compared to some recent ignition delay time measurements of H2/O2 mixtures. A strong dependence of the ignition delay time on the pressure and the NO2 concentration was observed, whereas the variation in the equivalence ratio did not exhibit any appreciable effect on the delay time. A mechanism combining recent H2/O2 chemistry and a recent high-pressure NOx sub-mechanism with an updated reaction rate for H2 + NO2 ⇄ HONO + H was found to represent correctly the experimental trends over the entire range of conditions. A chemical analysis was conducted using this mechanism to interpret the experimental results. Ignition delay time data with NO2 and other NOx species as additives or impurities are rare, and the present study provides such data over a relatively wide pressure range.

Caveats for Using Shock Tube in Blast-Induced Traumatic Brain Injury Research

Abstract: Blast-induced traumatic brain injury (TBI) is currently an important and very “hot” research topic because it has been acknowledged to be a significant source of morbidity and disability during the wars in Iraq and Afghanistan, among blast victims. A total of 545 academic articles about blast TBI research have been published since 1946, of which 82% (447 articles) have been published since 2003, and 57% (312 articles) were published from 2010 to 2013. A number of experimental models are currently implemented to investigate the mechanisms of blast-induced TBI in rodents and larger animals such as rabbits and swine. As the fundamental shock wave generator, shock tubes (either compressed air-driven or detonation-driven) are generally employed in these experimental models. The compressed air-driven shock tube is a horizontally mounted, circular steel tube, in which a gas at low pressure (the driven gas) and a gas at high pressure (the driver gas) are separated using diaphragms (such as polyester Mylar membrane). After the diaphragm suddenly ruptures at predetermined pressure thresholds (e.g., 126–147 kPa), shock waves are generated and propagate through the low pressure section (the driven section) toward the mouth of the shock tube. The detonation-driven shock tube is a cylindrical metal tube that is closed at one end. The blast, causing the shock waves, is generated by detonation of an explosive charge in the closed end of the tube. Both compressed air-driven and detonation-driven shock tubes can produce blast shock waves to induce blast injuries in animals. However, because of their designs and structures, both shock tubes are not able to generate the Friedlander wave (an ideal form of a primary blast wave) that occurs when a powerful explosive detonates in a free field, without nearby surfaces that can interact with the wave. A series of complex shock waves are then generated following the lead shock wave (the original shock front), including reflected shock waves, a Mach stem, an unsteady turbulent jet, and rarefaction waves. These waves can cause sudden compression or rarefaction effects upon any object encountered in their motion path, and transfer kinetic energy to the object. Therefore, if an experimental animal is placed inside the shock tube, these complex pressure waves will cause more severe and complex injuries that are rarely observed in blast victims, thus leading to false-positive results in the studies of blast TBI mechanism.

Instability, mixing, and transition to turbulence in a laser-driven counterflowing shear experiment

Abstract: In a turbulence experiment conducted at the Omega Laser Facility [Boehly et al., Opt. Commun. 133, 495 (1997)]], regions of 60 mg/cc foam are separated by an aluminum plate running the length of a 1.6 mm shock tube. Two counter-propagating laser-driven shocks are used to create a high speed, ΔV=140 km/s shear flow environment, sustained for ∼10 ns, while canceling the transverse pressure gradient across the interface. The spreading of the aluminum by shear-instability-induced mixing is measured by x-ray radiography. The width of the mix region is compared to simulations. Reynolds numbers ≳4×105 are achieved within the layer. Following the onset of shear, we observe striations corresponding to the dominant mode growth and their transition through non-linear structures to developed turbulence.

On the High-Temperature Combustion of n-Butanol: Shock Tube Data and an Improved Kinetic Model

Abstract: The combustion of n-butanol has received significant interest in recent years, because of its potential use in transportation applications. Researchers have extensively studied its combustion chemistry, using both experimental and theoretical methods; however, additional work is needed under specific conditions to improve our understanding of n-butanol combustion. In this study, we report new OH time-history data during the high-temperature oxidation of n-butanol behind reflected shock waves over the temperature range of 1300–1550 K and at pressures near 2 atm. These data were obtained at Stanford University, using narrow-line-width ring dye laser absorption of the R1(5) line of OH near 306.7 nm. Measured OH time histories were modeled using comprehensive n-butanol literature mechanisms. It was found that n-butanol unimolecular decomposition rate constants commonly used in chemical kinetic models, as well as those determined from theoretical studies, are unable to predict the data presented herein. Theref...

Spherical combustion clouds in explosions

Abstract: This study explores the properties of spherical combustion clouds in explosions. Two cases are investigated: (1) detonation of a TNT charge and combustion of its detonation products with air, and (2) shock dispersion of aluminum powder and its combustion with air. The evolution of the blast wave and ensuing combustion cloud dynamics are studied via numerical simulations with our adaptive mesh refinement combustion code. The code solves the multi-phase conservation laws for a dilute heterogeneous continuum as formulated by Nigmatulin. Single-phase combustion (e.g., TNT with air) is modeled in the fast-chemistry limit. Two-phase combustion (e.g., Al powder with air) uses an induction time model based on Arrhenius fits to Boiko’s shock tube data, along with an ignition temperature criterion based on fits to Gurevich’s data, and an ignition probability model that accounts for multi-particle effects on cloud ignition. Equations of state are based on polynomial fits to thermodynamic calculations with the Cheetah code, assuming frozen reactants and equilibrium products. Adaptive mesh refinement is used to resolve thin reaction zones and capture the energy-bearing scales of turbulence on the computational mesh (ILES approach). Taking advantage of the symmetry of the problem, azimuthal averaging was used to extract the mean and rms fluctuations from the numerical solution, including: thermodynamic profiles, kinematic profiles, and reaction-zone profiles across the combustion cloud. Fuel consumption was limited to $$\sim $$ 60–70 %, due to the limited amount of air a spherical combustion cloud can entrain before the turbulent velocity field decays away. Turbulent kinetic energy spectra of the solution were found to have both rotational and dilatational components, due to compressibility effects. The dilatational component was typically about 1 % of the rotational component; both seemed to preserve their spectra as they decayed. Kinetic energy of the blast wave decayed due to the pressure field. Turbulent kinetic energy of the combustion cloud decayed due to enstrophy $$\overline{\omega ^{2}} $$ and dilatation $$\overline{\Delta ^{2}} $$ .

An experimental investigation of the turbulent mixing transition in the Richtmyer-Meshkov instability

Abstract: The Richtmyer–Meshkov instability (RMI) is experimentally investigated in a vertical shock tube using a broadband initial condition imposed on an interface between a helium–acetone mixture and argon ( ). The interface is created without the use of a membrane by first setting up a flat, gravitationally stable stagnation plane, where the gases are injected from the ends of the shock tube and exit through horizontal slots at the interface location. Following this, the interface is perturbed by injecting gas within the plane of the interface. Perturbations form in the lower portion of this layer due to the shear between this injected stream and the surrounding gas. This shear layer serves as a statistically repeatable broadband initial condition to the RMI. The interface is accelerated by either a or planar shock wave, and the development of the ensuing mixing layer is investigated using planar laser-induced fluorescence (PLIF). The PLIF images are processed to reveal the light-gas mole fraction by accounting for laser absorption and laser-steering effects. The images suggest a transition to turbulent mixing occurring during the experiment. An analysis of the mole-fraction distribution confirms this transition, showing the gases begin to homogenize at later times. The scalar variance energy spectra exhibits a near inertial range, providing further evidence for turbulent mixing. Measurements of the Batchelor and Taylor microscales are made from the mole-fraction images, giving and 4 mm, respectively, by the latest times. The ratio of these scales implies an outer-scale Reynolds number of .

Manually operated piston-driven shock tube

Abstract: A simple hand-operated shock tube capable of producing Mach 2 shock waves is described. Performance of this miniature shock tube using compressed high pressure air created by a manually operated piston in the driver section of the shock tube as driver gas with air at 1 atm pressure as the test gas in the driven tube is presented. The performance of the shock tube is found to match well with the theoretically estimated values using normal shock relations. Applications of this shock tube named Reddy tube, include study of blast-induced traumatic brain injuries and high temperature chemical kinetics.

Formation of particle jetting in a cylindrical shock tube

Abstract: A dense, solid particle flow is numerically studied at a mesoscale level for a cylindrical shock tube problem. The shock tube consists of a central high pressure gas driver section and an annular solid powder bed with air in void regions as a driven section with its far end adjacent to ambient air. Simulations are conducted to explore the fundamental phenomena, causing clustering of particles and formation of coherent particle jet structures in such a dense solid flow. The influence of a range of parameters is investigated, including driver pressure, particle morphology, particle distribution and powder bed configuration. The results indicate that the physical mechanism responsible for this phenomenon is twofold: the driver gas jet flow induced by the shock wave as it passes through the initial gaps between the particles in the innermost layer of the powder bed, and the chaining of solid particles by inelastic collision. The particle jet forming time is determined as the time when the motion of the outermost particle layer of the powder bed is first detected. The maximum number of particle jets is bounded by the total number of particles in the innermost layer of the powder bed. The number of particle jets is mainly a function of the number of particles in the innermost layer and the mass ratio of the powder bed to the gas in the driver section, or the ratio of powder bed mass (in dimensionless form) to the pressure ratio between the driver and driven sections.

Ballistic Range Measurements of Stagnation-Point Heat Transfer in Air and in Carbon Dioxide at Velocities Up to 18,000 Feet Per Second

Abstract: A new technique for measuring heat-transfer rates on free-flight models in a ballistic range is described in this report. The accuracy of the heat-transfer rates measured in this way is shown to be comparable with the accuracy obtained in shock-tube measurements. The specific results of the present experiments consist of measurements of the stagnation-point heat-transfer rates experienced by a spherical-nosed model during flight through air and through carbon dioxide at velocities up to 18,000 feet per second. For flight through air these measured heat-transfer rates agree well with both the theoretically predicted rates and the rates measured in shock tubes. the heat-transfer rates agree well with the rates measured in a shock tube. Two methods of estimating the stagnation-point heat-transfer rates in carbon dioxide are compared with the experimental measurements. At each velocity the measured stagnation-point heat-transfer rate in carbon dioxide is about the same as the measured heat-transfer rate in air.

Detailed Simulations of Shock-Bifurcation and Ignition of an Argon-diluted Hydrogen/Oxygen Mixture in a Shock Tube

Abstract: Detailed simulations of the bifurcation and ignition of an Argon-diluted Hydrogen/Oxygen mixture in the two-stage weak ignition regime are performed. An adaptive meshrefinement (AMR) technique is employed to resolve all relevant physical scales that are associated with the viscous boundary-layer, the reaction front, and the shock-wave. A high-order hybrid WENO/central-differencing method is used as spatial discretization scheme, and a detailed chemical mechanism is employed to describe the combustion of the H2/O2 mixture. The operating conditions considered in this study are p5 = 5 bar and T5 = 1100 K, and fall in the third explosion limit. The computations show that the mixing of the thermally stratified fluid, carrying different momentum and enthalpy, introduces inhomogeneities in the core-region behind the reflected shock. These inhomogeneities act as localized ignition kernels. During the induction period, these kernels slowly expand and eventually transition to a detonation wave that rapidly consumes the unburned mixture. In competition with this detonation wave are the presence of secondary ignition kernels that appear in the unreacted core-region between reflected shock and detonation wave.

CO concentration and temperature measurements in a shock tube for Martian mixtures by coupling OES and TDLAS

Abstract: CO concentration and gas temperature distribution are diagnosed behind a strong shock wave simulating the Martian atmosphere entry processes by coupling optical emission spectroscopy (OES) and tunable diode laser absorption spectroscopy (TDLAS). The strong shock wave (6.31 ± 0.11 km/s) is established in a shock tube driven by combustion of hydrogen and oxygen. Temperature of the shock-heated gas is inferred through a precise analysis of the high temporal and spatial resolution experimental spectral of CN violet system (B 2 Σ + →X 2 Σ +, Δv = 0 sequence) using OES. A CO absorption line near 2,335.778 nm is utilized for detecting the CO concentration using scanned-wavelength direct absorption mode with 50 kHz repetition rate. Combined with temperature results from OES, CO concentration in the thermal equilibrium region is derived. The current experimental results are complementary for determining an accurate rate coefficient of CO2 dissociation and validation relevant chemical kinetics models in Mars atmosphere entry processes.

High temperature shock tube and theoretical studies on the thermal decomposition of dimethyl carbonate and its bimolecular reactions with H and D-atoms.

Abstract: The shock tube technique was used to study the high temperature thermal decomposition of dimethyl carbonate, CH3OC(O)OCH3 (DMC). The formation of H-atoms was measured behind reflected shock waves by using atomic resonance absorption spectrometry (ARAS). The experiments span a T-range of 1053–1157 K at pressures ∼0.5 atm. The H-atom profiles were simulated using a detailed chemical kinetic mechanism for DMC thermal decomposition. Simulations indicate that the formation of H-atoms is sensitive to the rate constants for the energetically lowest-lying bond fission channel, CH3OC(O)OCH3 → CH3 + CH3OC(O)O [A], where H-atoms form instantaneously at high temperatures from the sequence of radical β-scissions, CH3OC(O)O → CH3O + CO2 → H + CH2O + CO2. A master equation analysis was performed using CCSD(T)/cc-pv∞z//M06-2X/cc-pvtz energetics and molecular properties for all thermal decomposition processes in DMC. The theoretical predictions were found to be in good agreement with the present experimentally derived rat...

Shock Waves in Dispersive Eulerian Fluids

Abstract: The long-time behavior of an initial step resulting in a dispersive shock wave (DSW) for the one-dimensional isentropic Euler equations regularized by generic, third-order dispersion is considered by use of Whitham averaging. Under modest assumptions, the jump conditions (DSW locus and speeds) for admissible, weak DSWs are characterized and found to depend only upon the sign of dispersion (convexity or concavity) and a general pressure law. Two mechanisms leading to the breakdown of this simple wave DSW theory for sufficiently large jumps are identified: a change in the sign of dispersion, leading to gradient catastrophe in the modulation equations, and the loss of genuine nonlinearity in the modulation equations. Large amplitude DSWs are constructed for several particular dispersive fluids with differing pressure laws modeled by the generalized nonlinear Schrodinger equation. These include superfluids (Bose–Einstein condensates and ultracold fermions) and “optical fluids.” Estimates of breaking times for smooth initial data and the long-time behavior of the shock tube problem are presented. Detailed numerical simulations compare favorably with the asymptotic results in the weak to moderate amplitude regimes. Deviations in the large amplitude regime are identified with breakdown of the simple wave DSW theory.

Investigation of the initial perturbation amplitude for the inclined interface Richtmyer?Meshkov instability

Abstract: A simulation studying the effects of inclination angle and incident shock Mach number on the inclined interface Richtmyer–Meshkov instability is presented. Interface inclination angle is varied from 30° to 85°, with incident shock Mach numbers of 1.5, 2.0 and 2.5 for an air over SF6 interface. The simulations were performed in support of experiments to be performed in the Texas A&M shock tube facility, and were created with the ARES code developed at Lawrence Livermore National Laboratory. The parametric cases are separated by inclination angle into nonlinear and linear initial perturbation cases. A linear initial perturbation is defined as when the interface amplitude over wavelength is less than 0.1. Density, pressure gradient and vorticity plots are presented for a nonlinear and a linear case to highlight the differences in the flow field evolution. It is shown that the nonlinear case contains strong secondary compressible effects which reverberate through the interface until late times, while in the linear case these waves are almost completely absent. The inclined interface scaling method presented in previous work (McFarland et al 2011 Phys. Rev. E 84 026303) is tested for its ability to scale the mixing width growth rate for linear initial perturbation cases. This model was shown in the previous work to collapse data well for varying Mach numbers and nonlinear inclination angles. The scaled data is presented to show that a regime change occurs in the mixing width growth rate near an inclination angle of 80° which corresponds to the transition from a linear to nonlinear initial perturbation.